1. Field of the Invention
The present invention relates generally to high-power electromagnetic devices, and more particularly to an integral convection cooling system for improving the efficiency of such devices during operation.
2. Description of the Related Art
Inductors represent a large class of electromagnetic devices. The simplest inductor, a solenoid, is merely a coil of wire, ordinarily wound around a core material. Current flowing through the wire creates a magnetic field within the core; when a voltage is applied across the inductor, the magnetic field causes the current to rise as a ramp, the slope of which depends on the strength, or inductance, of the device. Single-coil inductors are used, for example, in many RF and tuned circuits.
The core of an inductor may be no more than a hollow tube. However, winding the wire around a magnetic material augments the magnetic field within the inductor, and therefore multiplies the inductance of the coil by the material's magnetic permeability.
Closely coupling two coils results in a transformer. An AC voltage applied to a first, or primary coil appears across the other, or secondary coil at an altered level determined by the ratio of wire turns in the primary and secondary coils. In transformers, the coils are frequently wound around different portions of the same core, resulting in maximum coupling between the windings. Transformers are used to change an input voltage value to a different value for use in a particular application, and also serve to isolate electronic devices from their power sources.
Electricity supplied over long distances must ordinarily be provided at high voltage levels due to power losses in transmission. Large power transformers situated near delivery points are utilized to bring the voltage down to standard line levels. These transformers operate at very high power levels, typically in the megawatt range. The performance of such devices is necessarily limited by the temperature rise they experience, as well as by the magnetic saturation of the core. A typical high-voltage power transformer exhibits a maximum temperature tolerance of 110.degree. C., and a maximum core saturation value of 20,000 Gauss.
Transformers generate heat through energy losses. A portion of input power is inevitably dissipated in the core, the windings, and the dielectric materials that insulate the windings, increasing the temperature of the transformer's environment. This, in turn, results in elevated resistance within the windings (which are generally copper), increased hysteresis losses within the core, decreased saturation magnetization of the core, and degradation of the transformer's insulation. Ultimately, these factors can lead to significant and permanent efficiency reductions.
To inhibit excessive temperature rise, high-voltage power transformers are usually cooled by surrounding them with oil. The final, steady-state temperature of the transformer reflects an equilibrium between power losses and the heat-dissipation properties of the oil. As the oil is heated it experiences a decrease in density; accordingly, oil in contact with the transformer coils absorbs the greatest amount of heat and, as a result, becomes least dense and rises relative to the surrounding oil. As the rising oil makes contact with the walls of the housing it transfers heat thereto (and, ultimately, with the transformer's exterior environment), cooling and increasing in density. The cooled oil travels toward the bottom of the container, replacing heated oil rising from the windings. This natural convection, caused by the interplay of gravity and heat-induced density variations, represents the cooling mechanism most commonly utilized in commercial high-voltage power transformers.
Unfortunately, the gravitational forces that circulate the oil are relatively weak. Temperature gradients across oil reservoirs are often observed to be quite large, signifying relatively poor heat transfer. Transformer windings frequently develop "hot spots"--regions of intense heating due to ineffective cooling--that can cause insulation to quickly break down.
To improve the efficiency of heat dissipation, transformers are frequently equipped with cooling fixtures (e.g., fins) on the outside of the transformer housing, and occasionally with pumping devices to circulate the oil within the housing. However, because oil pumps are cumbersome, consume power and require maintenance, they are not typically employed.